1. Field of the Invention
The present invention relates to a control system for a transistor inverter, and more particularly to a control system for an inverter apparatus which is suitable for a power source for supplying an oscillating current (high frequency current) to an induction or heating coil or the like in an electromagnetic induction heating apparatus.
2. Description of the Prior Art
As to a power source apparatus of this kind, there has been conventionally used a transistor inverter as shown in FIG. 1. In FIG. 1, reference numeral 1 denotes a thyristor rectifier for varying a d.c. voltage; 2, a choke coil; 3, an electrolytic capacitor used as a voltage source; 4a, 4b, 4c, 4d, transistors for performing high frequency switching; 5a, 5b, 5c, 5d, back-flow diodes; 6, a resonance capacitor for improving the power factor; and 7, a heating coil in the form of an induction coil.
In FIG. 1, the transistors 4a through 4d constituting an inverter 40 perform switching only in accordance with the LC resonance frequency defined by the resonance circuit composed of the resonance capacitor 6 and the heating coil 7. Accordingly, the load current I is controlled by changing a d.c. intermediate circuit voltage V.sub.DC in accordance with the phase control of the thyristor rectifier 1. Accordingly, the transistor inverter requires an arrangement for controlling the switching timing of the transistors 4a through 4d and an arrangement for controlling the phase of the thyristor rectifier 1, and therefore inevitably has a complicated arrangement. Moreover, the thyristor rectifier 1 has a disadvantage in that the power factor on the power supply side is deteriorated when the firing angle of the thyristor rectifier 1 is increased.
FIG. 2 shows a power source in which a diode rectifier 8 is used instead of the thyristor rectifier 1 shown in FIG. 1. Here, the load current I is controlled by controlling only the switching timing of the transistors 4a, 4b, 4c and 4d, while maintaining the d.c. intermediate voltage V.sub.DC constant. In FIG. 2, reference numeral 9 denotes a current transformer, which is coupled to the current path of the load current I, for detecting the load current I. Reference numeral 10 denotes a full wave rectifying circuit for rectifying the current detected by the current transformer 9 over the full wave thereof after the detected current is converted into a voltage signal. Reference numeral 11 denotes a current adjusting circuit for controlling the load current I by controlling the output voltage from the full wave rectifying circuit 10 in accordance with a load current instruction value IK. Reference numeral 12 denotes a V/F converter for converting the voltage output from the current regulator 11 into a corresponding frequency output so as to obtain a base drive signal for the transistors 4a, 4b, 4c and 4d.
In the above-mentioned control system, its load circuit is formed by an L-C-R series resonance circuit, and thus this control system has the following relationships: ##EQU1## This system utilizes such a fact that the load current I varies in accordance with the change in frequency f. That is, in such a circuit, the output frequency f of the inverter 40 is changed in accordance with the desired load current I.
FIG. 3 shows the relationship between the load current I and the output frequency f of the inverter 40.
FIG. 4 illustrates the relationship between the output voltage Vo from the inverter 40 and the load current I. It is noted that the output voltage Vo of the inverter 40 has a leading phase of.gamma. with respect to the load current I in this control system, as shown in FIG. 4. That is, the transistors 4a and 4d are interrupted before the load current drops to zero, and subsequently the transistors 4b and 4c (the transistors opposite the transistors 4a and 4d) are turned on, respectively. The reason follows. After the transistors through which the load current I flows are interrupted and if subsequently the transistors opposite the other transistors are turned on under a condition that the load current is flowing in the reverse direction after the load current is rendered to zero, the reverse recovery current of the back-flow diodes 5a, 5b, 5c, and 5d would increase, so that the heat generation of the back-flow diodes becomes large, particularly for a high frequency application, as with the above-mentioned apparatus. Therefore, there is the possibility that the transistors would be damaged. Furthermore, a switching loss would inevitably occur when the opposite transistors are turned on.
Because of the above-mentioned reasons, it is required that the output voltage Vo of the inverter 40 have a leading phase with respect to the load current I. As a consequence, the inverter is required to be controlled at a frequency higher than the resonant frequency of the L-C-R circuit in the case of the frequency control system as shown in FIG. 2.
However, in the case of using a coil L, as in the case of induction heating, an object to be heated has a magnetic permeability and a specific resistance which vary greatly, depending upon its temperature, and thus the resonance frequency and resonance current vary depending upon the temperature of the heated object.
FIG. 3 shows examples of the above-mentioned relationship between the resonance frequency and the resonance current, in which curve A relates to the object to be heated at a temperature around room temperature, while curve B relates to the object to be heated at a temperature exceeding the Curie temperature. From the above-mentioned reason, the frequency control as shown in FIG. 2 is not effective in the case of a frequency below f.sub.B shown in FIG. 3, and therefore, there is a disadvantage in that power cannot be effectively supplied to the object to be heated at a temperature around room temperature, as shown by curve A.